Course Profile - Curriculum Services Canada

Teacher/Learning Strategies
Act 1.3.1 Student Activity: Students generate their own notes from a lesson on the equilibrium constant
for slightly soluble ionic substances and its applications in the environment, society and industry. They
investigate possible related careers. Students set up their lab for Act. 1.3.2 and, based on the lab
procedure provided by the teacher, prepare their own observation tables. Students assess each other’s
lesson notes outside class time.
Teacher Facilitation: To provide a context for learning the solubility product constant, the teacher
refers to applications of equilibrium solubility such as kidney stones, dissolved gases in the circulatory
system of divers, the use of barium sulfate in medical diagnosis, the formation of stalactites and
stalagmites as found in the Bonnechere Caves in Ontario, hard water and water softeners, refining nickel,
developing camera film, and the massive deposits of limestone and gypsum as oceans dry up. A more
detailed treatment of these applications can occur in Act 1.3.5 once students have mastered the concept
of K sp and the common ion effect. Based on the results of the diagnostic quiz in Act 1.1.2, the teacher
uses the students’ prior knowledge of unsaturated, saturated and supersaturated solutions and links it to
chemical equilibrium systems. Students will apply this knowledge in Act 1.3.2 to determine an actual K sp
value. The teacher provides students with the lab procedure for Act 1.3.2.
1.3.2 Student Activity: Students conduct a lab to quantitatively determine the K sp value for a relatively
insoluble salt, e.g., silver acetate by displacement with copper wire, lead(II) chloride with a strip of zinc
metal.
Teacher Facilitation: The teacher could set up the analysis section of this lab to lead students step-bystep
to arrive at the expression and value for the K sp of silver acetate or lead(II) chloride. The teacher
may quickly check students’ observation tables.
1.3.3 Student Activity: Students solve K sp questions.
Teacher Facilitation: The teacher should choose K sp problems involving concentration, including molar
solubility of pure substances in water. Problems could be collected for evaluation.
1.3.4 Student Activity: Students take notes on the common ion effect, precipitation reactions, predicting
precipitate formation using K sp data and applications from a teacher-directed lesson. Students solve
problems involving common ion effect and compare their answers to posted solutions.
Teacher Facilitation: The teacher could do a quick demonstration of common ion effect such as how an
excess of ammonium ions reduces the concentration of hydroxide ions. To do so place a petri dish on an
overhead projector, and fill it halfway with 1mol/L ammonium hydroxide. Add two drops of
phenolphthalein. The solution should be red. Stir a little solid ammonium chloride into the solution; the
red colour will disappear. The ammonium ions suppress the ionization of the ammonium hydroxide. An
example of a practical use of equilibrium constants is in the use of barium sulfate as an X-ray contrast
medium. Doctors must make certain that almost no poisonous barium ions are present to dissolve in a
person’s body fluids. Knowing the solubility product constant for barium sulfate, calculations are made
to determine the amount of barium sulfate needed for the procedure. To ensure that the concentration of
barium ions will not exceed the safety level, a soluble salt such as sodium sulfate, which contributes
sulfate ions, is added to increase the sulfate ion concentration and reduce the barium ion concentration.
Because sulfate ion is present in both barium sulfate and sodium sulfate, sulfate is the common ion
present in both substances dissolved in the solution. Some students will have trouble grasping the
concept of the common ion effect. First, remind them that because the solubility of barium sulfate is very
low, it is not involved in the equation for the solubility product constant. It follows that the product of the
two ion concentrations is equal to the constant, K sp . A constant value cannot change, so if one of the
concentrations rises, the other must drop. By means of this principle, the common ion effect can be used
to increase or decrease the concentration of a chosen ion.
Unit 1 - Page 10
• Chemistry – University Preparation